1. Field of the Invention
This invention relates to apparatus for determining the freezing point of a liquid.
2. Prior Art
In our earlier Patent Specification No. 3,600,933 apparatus for the determination of the freezing point of a liquid is described comprising a probe for at least partial immersion in the liquid, means for vibrating said probe, means for cooling the probe and means responsive to the thickness of the solidified material deposited (i.e. solidified solvent in the case of a solution) on the probe and operative to control the cooling so as to maintain equilibrium conditions with a mantle of solidified material of constant thickness, and means for indicating or recording the temperature of the mantle.
When equilibrium conditions are obtained with a mantle of constant thickness, then the heat extracted by the cooling means is just sufficient to maintain the surface of the solid mantle at the freezing point. The vibration of the probe prevents supercooling. The abovedescribed apparatus has particular advantages in determining the freezing point of a solution. At the freezing point, latent heat is absorbed and there is a plateau or change in slope of the cooling curve (that is the relationship between temperature and time) as the liquid is gradually cooled. Particularly with a small sample of solution, no flat plateau is observed because, as increasing amounts of solvents freeze, the concentration of the remainder of the solution increases with a consequent decrease in the freezing point. The result is that what would have been a plateau for a pure solvent becomes a sloping line having a slope different from that of the liquid above its freezing point. The true freezing point of the solution is, in these circumstances, the temperature at which this change of the slope occurs. It is usually difficult to establish this from a graphical record and it is almost impossible to discover the change of slope in the presence of any significant degree of supercool. The above-described form of probe enables the freezing point to be determined. The probe forms a unit which can be put in the liquid in a sample cell or in a large tank and can operate automatically to provide an indication or record of the freezing point of the liquid.
In the apparatus described in the above-mentioned specification the probe itself is cooled and hence the mantle of solid material forms on the probe. It is the surface of the mantle which is at the exact temperature to be measured but this mantle will normally have a finite thickness and will extend over the temperature sensor. In order to maintain the mantle at constant thickness, heat is continuously extracted from the interior of the probe so that there is heat flow from the liquid through the mantle into the probe. In these circumstances therefore there is a slight error in the temperature sensed if the temperature sensor is beneath the surface of the mantle. This problem particularly arises with a form of probe in which, to sense the thickness of the mantle, an additional element is provided adjacent the probe so that the solid mantle, as it builds up on the probe, comes in contact with and attaches the probe to this additional element thereby changing the resonant frequency. The marked change in resonant frequency provides a very convenient means of determining when the mantle has reached a predetermined thickness. To measure the temperature at the surface of the mantle in this case, as described in the aforementioned specification, the temperature sensor may be mounted on the additional element.